Fix a typo in SelectableTextBlock.ClearSelection().

[VladiStep]

What does the pull request do?

It fixes a typo in "SelectableTextBlock.cs".

Checklist

(none of the following checkboxes can be checked, because it only changes the XML comment)

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Fixed issues

Fixes #13732

[VladiStep]

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[maxkatz6]

Thanks!